In astrophotography, producing sharp, round stars across the entire frame requires more than just high-quality optics—it also depends on precise calibration of the image plane.
This article outlines a step-by-step approach to the four essential components of image plane calibration:
>> Final Focus
Baseline Focus
Before calibration, the system must be accurately focused to establish a reliable baseline. This initial focus ensures that any subsequent adjustments are measured against the true optical axis.
For astrophotography, the reference point of focusing is typically the center of the image circle.
However, when field curvature is significant, a practical compromise is to set focus at about 1/3 of the image circle radius, balancing star quality between the center and the edges.
- Theoretical condition → Focus at center
- With field curvature → 1/3 of the image circle
▲ The 1/3 position of the image circle is shown in yellow
After each tilt or back focus correction in following steps, the focusing step must be repeated. Otherwise, test results may be misleading and lead to incorrect conclusions.
Sensor Tilt Adjustment
Sensor tilt occurs when your camera sensor is not perfectly perpendicular to the optical axis of the telescope. This can cause stars to appear sharp on one side of the frame but distorted on the opposite side.
Tilt issues become especially noticeable when using larger sensors (such as full-frame or medium-format) and fast optical systems (e.g., f/5 or faster), where the image circle is more sensitive to alignment errors.
Sensor tilt can stem from several sources:
- Uneven compression rings or poorly seated adapters
- Flex in the focuser or image train
- Sensor misalignment in the camera body
Even very small tilt angles—less than a millimeter—can visibly affect star shapes, especially near the corners of the image.
Identifying the tilt direction
With accurate focus already established, you can begin tilt diagnosis by capturing an image of a dense star field. Then carefully examine all four corners of the frame:
- Identify the area where stars appear roundest and best focused
- Look for regions where stars show elongation—this could appear as stretching either radially (sagittal) or tangentially
From this comparison, you can roughly map the position of the sensor tilt axis and infer whether the sensor is positioned too close to or too far from the focal plane in that specific region.
Example A
Stars in the lower right are round, while those in the upper left show strong sagittal (radial) elongation—indicating the sensor is too close in that area.
Example B
Stars at the top are sharp and round, while those in the lower left show strong tangential elongation—indicating the sensor is too far in that area.
When calibrating tilt, in addition to focusing at the center, you may also try focusing sequentially at each of the four corners. This approach makes it easier to observe how star shapes deform across opposite corners.
Correcting Solutions
Once tilt has been identified, the next step is mechanical correction. Many refractors on the market offer solutions for tilt adjustment, either through integrated components or optional accessories.
All William Optics refractors include a built-in sensor tilt corrector as part of the OTA. This allows for precise mechanical tilt correction without the need for external plates or adapters. For operation instructions, refer to the following guides:
For users of third-party imaging setups, tilt adapters from ZWO, QHY, and others offer similar functionality. These are typically mounted between the camera and the flattener or filter wheel and allow for small, precise adjustments via tensioning screws.
Whichever system you use, always make adjustments incrementally. Retest after each change using the same evaluation method—whether visual or software-based—to confirm improvements. This process may take several iterations but is essential for achieving flat, well-focused stars across the entire frame.
Software Analysis Tools
In addition to visual inspection, several software applications offer data-driven approaches to help determine sensor tilt more accurately. These tools can map star sharpness across the field and quantify deviation, offering a clearer picture of how to proceed with adjustments:
- ASTAP – Aberration Inspector ASTAP (Astrometric STAcking Program) includes a built-in “Aberration Inspector” that analyzes star shapes and sizes in the corners and center of the frame. It displays tilt and field curvature in a visual, quadrant-based layout. It supports FITS and a variety of camera file formats.
- CCD Inspector A commercial program designed to evaluate field curvature and tilt using 3D surface maps. It quickly reveals where the sensor deviates from ideal alignment. Note: a license is required.
- NINA – Hocus Focus Plugin Integrated into the NINA imaging suite, the Hocus Focus plugin generates full-frame FWHM maps. Paired with real-time focusing and “Image Grading” modules, it provides immediate feedback on tilt during acquisition—ideal for automated workflows.
- PixInsight – SubframeSelector + FWHMEccentricity Script Advanced users can utilize PixInsight scripts to analyze star eccentricity and resolution across the frame. These tools help isolate subtle tilt effects and compare multiple subframes systematically. Note: a license is required.
Back Focus Adjustment
If your image shows no signs of sensor tilt—or if tilt has already been corrected and precise focus has been achieved—but stars near the edges of the frame still appear soft or distorted, the likely cause is a back focus error. This step ensures your camera sensor is positioned at the correct distance from the telescope’s flattener or reducer. (For foundational information about this back focus, see our article: What is Back Focus?)
Common Causes of Back Focus Error
Even if your setup matches the recommended back focus distance on paper, small variances in the actual build can still cause image degradation. Typical causes include:
- Thread or adapter tolerances – small gaps between parts can slightly change spacing.
- Filter thickness – most glass filters shift focus slightly ( filter thickness ÷ 3 = additional spacing )
- Tilt or fine-adjustment adapters – may introduce tiny spacing changes when adjusted.
- Temperature – metal parts can expand or contract, affecting distance slightly.
- Flexure – heavy cameras or long setups may sag slightly when the scope points at different angles.
- Swapped components – replacing or rethreading parts (e.g. filter drawer, reducer) can change spacing by fractions of a millimeter.
These small differences can create soft corners or field curvature even in an otherwise “correct” setup, which is why visual or software-based verification after assembly is essential
Evaluating Back Focus Offset
Take a well-focused image of a dense star field and examine the shape of stars at the center and corners:
if the camera sensor is positioned too far from the optical plane, star shapes will appear elongated in the tangential direction—stretching around the center of the frame.
Conversely, if the sensor is too close, the stars will stretch in the sagittal (radial) direction—pointing toward or away from the center.
▲ Camera Sensor Too Far Away
▲ Camera Sensor Too Close
Tips
- If distortion is symmetrical and appears equally in all four corners, back focus is likely the primary issue rather than sensor tilt.
- These symptoms often become more pronounced with fast focal ratios or larger sensors. Be sure to review the image in high resolution and use a calibrated monitor if possible.
Solutions
Common solutions for adjusting back focus include:
- Spacers (Shims) Add or remove precision spacers to fine-tune the distance between the last optical surface and the camera sensor, which is the most common and stable way to adjust back focus. Some products are supplied with spacers, and they are available in various thicknesses (e.g., 0.1 mm, 0.3 mm, 0.5 mm, 1 mm), making them ideal for small, precise adjustments.
- Extension Tubes For larger back focus offsets, manufacturers may provide threaded extension tubes designed to match the required size and thread standard, allowing you to bridge the gap effectively.
- Built-in Adjustment Mechanisms Some optical systems integrate an adjustable back focus feature, allowing you to dial in the exact distance without changing accessories. For example, many William Optics flatteners include a built-in fine-tuning mechanism. For detailed instructions, see: Flattener Back Focus Adjustment.
Make small, incremental adjustments and verify after each change using the same visual or software-based evaluation method. Repeat as needed until star shapes are consistently round and sharp across the entire frame
Final Compromise Focus
Once sensor tilt and back focus have been optimized, the final step is to achieve critical and balanced focus. This process ensures that the focal plane is positioned exactly where the optical system delivers its sharpest performance.
A good general guideline for astrophotography is to set focus approximately one-third of the way from the center of the image circle toward the edge. This position balances sharpness between the center and corners, minimizing the impact of residual field curvature.
However, when the image plane is essentially flat, or back focus cannot be adjusted—as with many Petzval designs—you may instead shift the focus position toward the edge. This approach helps ensure that both the corner and central stars appear as perfectly round points.
Focus Position | 1/3 of the image circle radius | Near the Edge (Off-axis) |
Use Case | • Residual field curvature present
• Balanced sharpness required across field | • Image plane essentially flat
• No adjustable back focus available |
Optical Design | • Doublet/triplet refractors with flatteners
• RC or SCT systems with moderate curvature | • Petzval refractors
• William Optics Pleiades (7-element) |
Object | • Deep sky object
• Planetary | • Wide star fields
• Deep sky object
• Mosaic imaging |
In summary, compromise focusing is adopted when residual field curvature or optical design limitations prevent perfect sharpness across the field. By selecting either one-third of the image circle or shifting closer to the edge, you can balance image quality and achieve more uniform star shapes from center to corner.